Method of making an electrolyte for an electrochemical cell

ABSTRACT

Described is a thin-film battery, especially a thin-film microbattery, and a method for making same having application as a backup or primary integrated power source for electronic devices. The battery includes a novel electrolyte which is electrochemically stable and does not react with the lithium anode and a novel vanadium oxide cathode Configured as a microbattery, the battery can be fabricated directly onto a semiconductor chip, onto the semiconductor die or onto any portion of the chip carrier. The battery can be fabricated to any specified size or shape to meet the requirements of a particular application. The battery is fabricated of solid state materials and is capable of operation between -15° C. and 150° C.

This invention was made with Government support under Contract No.DE-AC05-84OR21400 awarded by the U.S. Department of Energy to MartinMarietta Energy Systems, Inc. The Government has certain rights in thisinvention.

This application is a division of application Ser. No. 07/921,538 filedJul. 29, 1992 now U.S. Pat No. 5,338,625.

BACKGROUND OF INVENTION

1. Field of Invention

The invention is directed to a thin-film battery and a method for makingsame. More particularly, the invention is directed to a new thin-filmlithium battery having a novel electrolyte permitting a battery to befabricated having greatly enhanced energy density and specific energyover conventionally available batteries. The invention is also directedto a novel cathode permitting a battery to be fabricated havingsignificantly enhanced energy densities over conventionally availablebatteries.

2. Description of Prior Art

A battery is one of two kinds of electrochemical devices that convertthe energy released in a chemical reaction directly into electricalenergy. In a battery, the reactants are stored close together within thebattery itself, whereas in a fuel cell the reactants are storedexternally. The attractiveness of batteries as an efficient source ofpower is that the conversion of chemical energy to electrical energy ispotentially 100% efficient although the loss due to internal resistanceis a major limiting factor. This potential efficiency is considerablygreater than the conversion of thermal energy to mechanical energy asused in internal combustion engines, which always results in heattransfer losses. Moreover, the additional disadvantages of contaminantsemitted into the atmosphere as byproducts of incomplete combustion anddwindling availability of fuel supplies have intensified research intobatteries as an alternative source of energy.

One limitation of conventional batteries is that they use toxicmaterials such as lead, cadmium, mercury and various acid electrolytesthat are facing strict regulation or outright banning as manufacturingmaterials. Another limitation is that the amount of energy stored and/ordelivered by the battery is generally directly related to its size andweight. At one end of the development spectrum, automobile batteriesproduce large amounts of current but have such low energy densities andspecific energies due to their size and weight and such relativelylengthy recharge times that their usage as a source of propulsion isimpractical. At the other end of the development spectrum, small, light,lithium batteries used to power small electronic appliances andsemiconductor devices have much higher energy densities and specificenergies but have not had the capability to be scaled up to provide thehigh energy for high power applications such as use in automobiles.Further, these small, light, lithium batteries have low charge-dischargecycle capability, limited rechargeability and, even where scaled downfor microelectronics applications, size that frequently is many timeslarger than the semiconductor chip on which they are used.

Thin-film battery technology is foreseen as having several advantagesover conventional battery technology in that battery cell components canbe prepared as thin, e.g. 1 micron, sheets built up in layers usingtechniques common to the electronics industry according to the desiredapplication. The area of the sheets can be varied from sizes achievablewith present lithographic techniques to a few square meters providing awide range in battery capacity. Deposition of thin films places theanode close to the cathode resulting in high current density, high cellefficiency and a great reduction in the amount of reactants used. Thisis because the transport of ions is easier and faster in thin filmlayers since the distance the ions must move is lessened.

Most critical to battery performance is the choice of electrolyte. It isknown that the principle limitation on rechargeability of priorbatteries is failure of the electrolyte. Battery failure after a numberof charge-discharge cycles and the loss of charge on standing is causedby reaction between the anode and the electrolyte, e.g. attack of thelithium anode on the lithium electrolyte in lithium batteries. An extraprocess step of coating the anode with a protective material adds to thecomplexity, size and cost of the battery.

The power and energy density of a battery is also dependent upon thenature of the cathode. To achieve optimum performance, the open circuitvoltage and current density on discharge should be as high as possible,the recharge rate should be high and the battery should be able towithstand many charge-discharge cycles with no degradation ofperformance. The vanadium oxide cathode of the present invention has amuch higher capacity per mole than the crystalline TiS₂ of prior artcathodes.

The present invention avoids the limitations of present battery designand provides a novel battery having application as a battery used withmanufacture of semiconductor components and as a high energy, highcurrent macrobattery with appropriate scale-up of the describedprocesses. The present invention includes a novel electrolyte having agood conductivity but more importantly it has electrochemical stabilityat high cell potentials and requires no protective layer between it andthe anode during battery fabrication or use. The present invention alsoincludes a novel cathode having a microstructure providing excellentcharge/discharge properties.

SUMMARY OF THE INVENTION

A primary object of invention is to provide a new thin-film battery anda method for making same.

A second object of invention is to provide a new electrolyte for athin-film battery in which the electrolyte has good ionic conductivityand is not reactive with the battery anode.

Another object of invention is to provide a method for making animproved electrolyte for thin-film battery.

A yet further object of invention is to provide a new cathode havingimproved microstructure for a thin-film battery and a method for makingsame.

These and other objects are achieved by depositing a pair of currentcollecting films on a substrate; depositing an amorphous cathode layeron the larger of the two collecting films; depositing an amorphouslithium phosphorus oxynitride electrolyte layer over the cathode; anddepositing a metallic anode layer over the electrolyte.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a thin-film battery deposited onto asemiconductor chip package with current leads extending to asemiconductor chip.

FIGS. 2A-2D illustrates the layers in plan view to form a thin-filmbattery according to the present invention.

FIG. 3 schematically illustrates a cross-sectional view of a thin-filmbattery made according to the present invention.

FIG. 4A is a micrograph of a vanadium oxide cathode formed by asputtering process where the target is aged due to prior sputtering andthe process gas flow rate is less than about 15 sccm.

FIG. 4B is a micrograph of a vanadium oxide cathode formed by asputtering process where the target is fresh and the process gas flowrate is greater than about 15 sccm.

FIG. 5 illustrates the charge-discharge performance for a microbatterymade according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

There are many possible uses for a thin-film, rechargeable battery as aprimary or standby power source for low current electronic devices. Athin-film cell could be fabricated directly onto the semiconductor chip,the chip package or the chip carrier and could be fabricated to anyspecified size or shape to meet the requirements of a particularapplication. Referring to FIG. 1, a possible application is shown inwhich a thin-film cell 10 is deposited onto a semiconductor chip package12 with current leads 14 extending to the chip 16. A Li-VO_(x) cellabout 8 microns thick occupying an area of 1 square centimeter as shownhas a capacity of 130 microAmp-hours and could supply a current of up to100 microAmps at a voltage ranging from 3.7 volts at full charge toabout 1.5 volts near the end of its discharge. If a larger battery weredeposited over the unused area of the package, the capacity and currentdensity of the battery could of course be increased.

With reference to FIGS. 2A-D, the steps in fabricating such a singlecell are shown. Two current collectors, vanadium for example, aredeposited as a larger and a smaller 0.5 micron thick film, 18 and 20respectively, on a substrate 22 such as glass, alumina, sapphire orvarious semiconductor or polymer materials. The films may be depositedby rf or dc magnetron sputtering or diode sputtering of vanadium inArgon, vacuum evaporation or other such film deposition techniquescommon to the semiconductor electronics industry. Similarly, anamorphous vanadium-oxide, VO_(x), cathode 24 is deposited as a 1 micronthick film over the larger current collector 18 by sputtering vanadiumin Argon+14%O₂. An amorphous oxynitride lithium electrolyte film 26 isthen deposited over the cathode 24 by sputtering of Li₃ PO₄, lithiumorthophosphate, in 20 milliTorr of N₂ and a total gas flow of 14 sccm.As before, various film deposition techniques may be used forfabrication of the vitreous electrolyte film 26 although reactive DCsputtering is not available when lithium orthophosphate is the target asit is an insulator material and would accumulate charge until thedeposition process stopped. Example targets for the describedmicrobattery measured 25 millimeters in diameter by 3 millimeters thickand were prepared by cold pressing lithium orthophosphate powderfollowed by sintering of the pressed disc in air at 900° C. Depositionof a 1 micron thick film was carried out over a period of 16-21 hours atan average rate of 8-10 Angstroms per minute. The film 26 has thecomposition Li_(x) PO_(y) N_(z) where x has the approximate value of2.8; 2y+3z has the approximate value of 7.8; and z has the approximatevalue of 0.16 to 0.46. Deposition of a film 28 of lithium over thevitreous electrolyte film 26, the intervening substrate 22 and thesmaller current collector 20 completes the cell. A typical filmthickness for the lithium film 28 is about 5 microns. FIG. 3 is aschematic cross-section view of FIG. 2D.

Example performance characteristics of such a battery as described aboveare an open circuit voltage of 3.6 to 3.8 volts and, for a 1 micronthick cathode, a capacity of about 130 microAmp-hours per squarecentimeter for a discharge to 1.5 volts. The battery is capable ofproducing a discharge current of up to 2 milliAmps per square centimeterand can be recharged at a current of at least 20 microAmps per squarecentimeter. The battery has been subjected to more than 100charge/discharge cycles with no degradation in performance and, afterthe first few cycles, the efficiency of the charge/discharge process wasapproximately 100% Further, the vitreous oxynitride lithium electrolyte26 has demonstrated long-term stability in contact with the lithiumanode 28 such that the battery does not require the extra protectivefilm, typically lithium iodide, to prevent reaction of the lithium anodewith the electrolyte.

Performance of thin-film batteries has been critically limited by theproperties of the chosen electrolyte. For rechargeable lithiumbatteries, the electrolyte should have a high lithium ion conductivityand it must be chemically stable in contact with lithium. Filmsdeposited by sputtering or evaporation of inorganic compounds ontosubstrates held at ambient temperatures are usually amorphous. This isadvantageous because, for many lithium compounds, the lithium ionconductivity of the amorphous phase is orders of magnitude higher thanthat of the crystalline phase and the conductance of the amorphous filmis often adequate for use an as electrolyte. As many of these amorphousmaterials have acceptable low electronic conductivities, there is a widechoice of materials available for possible application in thin-filmcells which meet the first two requirements. However, instability incontact with lithium eliminates many materials from consideration andhas limited development of a thin-film lithium cell. The amorphouslithium phosphorus oxynitride film 26 of the present invention is madeby sputtering Li₃ PO₄ in pure N₂ and has both the desired electricalproperties and the stability in contact with lithium for fabrication ofelectrochemical cells.

A comparison of the conductivities at 25° C. for several electrolytecompositions in the lithium phosphosilicate system achieved bysputtering lithium silicates and lithium phosphates in Ar and Ar+0₂ isshown in Table 1. The lithium phosphosilicate listed had the highestconductivity of the films in the Li₂ 0:SiO₂ :P₂ O₅ system. Several ofthe more highly conductive lithium phosphosilicate films with differentcompositions were investigated as the electrolyte for lithium cells. Ineach case, the lithium anode 28 reacted with the electrolyte film 26.However, the electrolyte of the present invention was found to be stablein contact with the lithium anode although it contained only about 2 to6 at. % nitrogen. Moreover, as shown in Table 1, the conductivity ismore than 30 times greater than that of the film deposited by sputteringLi₃ PO₄ in 40% O₂ in Argon. Incorporation of nitrogen into the thinfilms of the present invention increases conductivity at least fivetimes greater than similarly prepared films containing no nitrogen. Theincrease in conductivity is due to an increase in lithium ion mobilityrather than an increase in the number of charge carriers brought aboutby a change in the structure of the electrolyte. Further, such cellsappear to be stable indefinitely, exhibiting only a small voltage losswhich is considered to occur due to the electronic conductivity of theelectrolyte.

                  TABLE 1    ______________________________________    Comparison of amorphous lithium phosphate, phosphosilicate, and    phosphorus oxynitride electrolyte films.                                     σ(25° C.)                         Film        × 10.sup.8                                            E.sub.2    Target   Process Gas Composition (S/cm) (eV)    ______________________________________    Li.sub.3 PO.sub.4             40% O.sub.2 in Ar                         Li.sub.2.7 PO.sub.3.9                                     7      0.68    Li.sub.3 PO.sub.4  +             40% O.sub.2 in Ar                         Li.sub.4.4 Si.sub.0.23 PO.sub.5.2                                     20     0.57    Li.sub.4 SiO.sub.4    Li.sub.3 PO.sub.4             N.sub.2     Li.sub.3.3 PO.sub.3.8 N.sub.0.22                                     240    0.56    ______________________________________

The enhanced conductivity, superior mechanical properties of nitridedglass(e.g. hardness, resistance to fracture) and chemical stability ofthe oxynitride lithium electrolyte of the present invention could alsobe used to fabricate enhanced electro-optic devices using electrochromiclayers, i.e. so called smart windows, because of the increasedresistance to attack from water vapor.

The performance of the lithium microbattery of the present invention isalso very dependent on formation of the cathode. Consideration of themicrostructure of the cathode is equally as important as considerationof the composition. Typical of prior thin-film batteries is the usecathodes having a characteristic crystalline microstructure. Themicrostructure is dependent on substrate temperature, extent of theerosion of the target material due to prior sputtering and the pressureand composition of the process gas during deposition. At substratetemperatures of 400° C., vanadium oxide cathodes, for example, consistof crystalline platelets standing on edge while films deposited ontosubstrates at about 50° C. consist of clusters of crystalline fibrousbundles. With reference to FIG. 4, two distinct types of microstructureare shown for vanadium oxide films deposited by reactive sputtering ofvanadium. When deposited from an eroded target, the cathode films 28were characterized by a high density of micron-sized fibrous clusters inFIG. 4A of crystalline V₂ O₅. When a fresh target surface is used andthe flow rate is increased to about 20 sccm, the microstructure of thecathode 28 has the smooth microstructure shown in FIG. 4B. The advantageachieved with the amorphous structure over the crystalline structure isthat at least three times more lithium ions can be inserted into cathode28 having such amorphous structure, thus resulting in a lithium cell ofmuch higher capacity.

As the sputtering target, e.g. vanadium, ages, the microstructure of thefilms deposited with higher flow rates gradually evolves to that of thefilms having fibrous clusters characteristic of deposition at the lowerflow rates. This change in the films is evident by a decrease insputtered target voltage (at constant power) and as much as a 30%decrease in deposition rate.

Lithium cells fabricated with crystalline or amorphous vanadium oxidecathodes had open circuit voltages of 3.6 to 3.7 volts. However,compared with amorphous cathodes, the rates of discharge and charge thatthe cells with the crystalline cathodes could sustain without excessivepolarization are significantly lower, usually less than 3 microAmps persquare centimeter. This probably results from poor transport across theinterface between the electrolyte 26 and the cathode 28 since theelectrolyte 26 does not conformably coat the fibrous clusters of thecrystalline cathode 28 but rather covers just the top portion, resultingin a relatively small contact area.

Lithium cells made according to the present invention having the lithiumphosphorus oxynitride electrolyte 26 and the smooth amorphous cathode 28may be discharged at rates of up to 3 milliAmps per square centimeter.With reference to FIG. 5, a set of charge-discharge curves for one cycleof such a cell is shown. The total charge passed through this cellbetween 3.64 volts and 1.5 volts is about 575 milliCoulombs. Thecapacity of the cell over this voltage range is 130 microAmp-hours persquare centimeter with an energy density of 1.2×10⁶ Joules per kilogrambased on combined masses of the cathode, electrolyte and anode.

The greatly enhanced energy density achievable with thin-film batteriesmade according to the present invention may, with suitable scaling ofprocess parameters, permit fabrication of high energy thin-filmmacrobatteries. For example, according to the present teachings, a25-kWh thin-film lithium battery could be constructed by connecting inseries approximately 46 large-area thin-film cells. Such a battery wouldhave an average voltage of 165 volts, a weight of 67 kilograms, a volumeof 36 liters, a specific energy of 370 Watt-hours per kilogram and anenergy density of 690 Watt-hours per liter.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A method for making an amorphous electrolyte foran electrochemical cell comprising the steps of:a) selecting asputtering apparatus chosen from the group consisting of rf magnetronsputterers and diode sputterers for deposition of thin films; b)selecting a lithium orthophosphate target material for sputtering insaid sputtering apparatus; c) selecting a pure Nitrogen process gas foroperation in said sputtering apparatus; d) operating said sputteringapparatus at a total gas pressure of 20 milliTorr and a total gas flowrate of at least 14 sccm; and e) depositing said electrolyte at anaverage rate of 8 Angstroms per minute.